Yarns have been developed which when woven or knitted into cloths are capable of preventing or dissipating electrostatic charges. Generally these yarns have been developed in response to the need for fabrics which are capable of dissipating or eliminating sparks either substantially or completely. Electrostatic discharges may result from many causes including the use of electronic equipment, interfabric friction, etc. It is particularly important, for example, to prevent discharge of electrical sparks in hospital operating rooms in which oxygen and the administration of highly flammable materials, such as anesthetic gases are almost always present. Electronic equipment is particularly susceptible to the introduction of error signals due to the presence of static discharge. Other obvious examples of the situations which should be kept free from static discharge through sparking are grain elevators, areas where explosives and fuel are stored or handles, etc. Thus, it will be appreciated that the needs for antistatic fabrics are legion.
One approach to the production of antistatic yarn and fabrics has been directed to blending textile material fibers with conductive metal fibers at the fiber drawing stage to produce a blend of the material fibers. In particular, it has been found to be desirable to produce a uniform and homogenous yarn and fabric in order to satisfy spark dissipation requirements. Moreover, it has been found that when the metal or other conductive fibers are dispersed uniformly throughout the yarn and fabric, the conductive fibers become entrapped within the textile fibers. When the conductive fibers are so uniformly dispersed, the fabric assumes a uniform appearance which is pleasing to the eye. Without such uniform distribution of the conductive fibers, the conductive fibers tend to cluster into small groups (or slubs) usually near the outer surface of the yarn. Where conductive fibers are of stainless steel or carbon materials, the slubs appear as numerous grey or black streaks in the finished fabric. Such slubbing problems were sought to be overcome by the invention disclosed in commonly assigned U.S. Pat. Nos. 3,703,073 and 3,828,543 to Goodbar et al, the disclosures of which are incorporated herein by reference.
The Goodbar et al. U.S. Pat. No. 3,703,073 relates to a method and apparatus for drawing a combination yarn consisting of textile fiber and metallic filaments. Broadly, the method comprises the simultaneous drawing of a textile fiber bundle and a multifilament metal bundle while guiding the metal bundle relative to the textile bundle to cause the latter continuously to cushion the metal bundle while controlling the tension forces upon the bundle of filaments such that a gradual breaking of filaments occurs. The broken filaments are distributed uniformly throughout the finished textile yarn. More specifically, the method employs feeding a textile fiber sliver into a first set of draw rolls while simultaneously feeding a metal multifilament bundle thereto which is guided into a continuous contact with the textile fiber bundle while controlling the tension forces thereon. The Goodbar et al. U.S. Pat. No. 3,828,543 is a divisional of the patent application from which the '073 patent issued and relates the antistatic yarn produced according to the method, and on the apparatus of the '073 patent.
In the Goodbar et al. patents, the metal fiber bundle is preferably in the form of a bundle of continuous multifilament stainless steel fibers in the form of a tow and includes a spool and purn arrangement wherein the spool is subjected to a rotational drag by applying tension on a strap and the continuous filament stainless steel tows are dispersed while under such tension.
Although the inventions of the Goodbar et al. patents successfully solved the problems theretofore existing in the production of such yarns and fabrics, later refinements of the invention were necessarily made which paved the way for further inventive contribution.
For example, more recently it became apparent that staple length stainless steel fibers in the form of a stainless steel "sliver" (as opposed to "tow") may be blended with textile fibers with perhaps, less difficulties and greater uniformity and homogeniety, particularly because of the effectively "pre-broken" character of the metal fibers. An example of such conductive fiber sliver is BEKINOX brand stainless steel multifiber "staple length" sliver marketed by Bekaert S. A., a Belgian corporation, in the form of rolls of bundles of staple length stainless steel fibers. When such staple length fibers were blended in the prior art system as disclosed in the Goodbar et al. patent, however, it was found that the staple length fibers, being discontinuous, were torn apart due to the tension applied, particularly between the location of unwinding from the spool and the point of entry into the back rolls of the apparatus, a distance usually greater than the length of the stainless steel staples. This resulted in a disruption of the operation and excessive waste of stainless steel sliver material. In addition, because the stainless steel sliver is wound on the spool progressively from left to right with respect to the axis of the spool and vice-versa, when it became unwound on the apparatus, when on the upright spindle of the prior art devices, the sliver bundle assumed an upward and downward movement which caused frictional contact and fraying between the portion of sliver being unwound and the surface of the portion of sliver which was still wound on the spool.
We have invented a method and apparatus which not only avoids the above-noted disadvantages of the prior art, but which may be utilized with conductive filament tows, metal or otherwise, either in the form of continuous multifilament bundles or staple length sliver bundles with the result that improved yarns and fabrics are produced.